Since silver halide-using photography is superior in photographic characteristics, e.g., sensitivity, facility of gradient control, etc., to other photographies such as electrophotography, diazo photography and so on, it has been extensively employed. In recent years, a technique has been developed which enables simple and rapid formation of images by changing the image-forming processing of the silver halide-using photography from a conventional wet process using a developing solution or the like to a dry process by heating or the like.
Heat developable photosensitive materials are well known in the field of the photographic art, and such materials and their processes are described in, for example, Shashin Kogaku no Kiso, pp. 553-555, Corona Company (1979); Eizo Joho, p. 40 (Apr. 1978); Nebletts Handbook of Photography and Reprography, 7th Ed., pp. 32 and 33, Van Nostrand Reinhold Company; U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Patent Nos. 1,131,108 and 1,167,777, and Research Disclosure, pp. 9-15 (RD-17029) (June, 1978).
Many methods for obtaining color images have been proposed. As for the method of forming color images by binding an oxidation product of a developing agent to a coupler, reducing agents of the p-phenylenediamine type and phenolic or active methylene-containing couplers are proposed in U.S. Pat. No. 3,531,286; reducing agents of the p-aminophenol type in U.S. Pat. No. 3,761,270; reducing agents of the sulfonamidophenol type in Belgian Patent No. 802,519 and Research Disclosure, pp. 31 and 32 (Sept., 1975); and combinations of reducing agents of the sulfonamidophenol type with 4-equivalent couplers in U.S. Pat. No. 4,021,240.
However, the above-described method suffers from the defect that it provides turbid color images because both reduced silver image and the color image are simultaneously produced in optically exposed areas after heat development. With the intention of solving this problem, a method have been proposed which involves the removal of the silver image by carrying out a liquid treatment and a method which involves the transfer of dyes alone into another layer, e.g., a sheet having an image-receiving layer. However, the latter method has the defect that it is not easy to differentiate between dyes and substances which have not yet undergone the reaction, and to only transfer the dyes.
Another method which comprises the introduction of a nitrogen-containing heterocyclyl group into a dye to form the silver salt thereof and the release of the dye from the silver salt by heat development is described in Research Disclosure, pp. 54-58 (RD-16966) (May 1978). This method is unsuitable for common use because it is difficult to inhibit the release of the dye in the unexposed part and, therefore, a clear image cannot be formed.
A further method which comprises formation of positive color images using the light-sensitive silver dye bleach process is described with regard to useful dyes and bleach processes in, e.g., Research Disclosure, pp. 30-32 (RD-14433) (Apr., 1976); ibid., pp. 14 and 15 (RD-15227) (Dec., 1976); U.S. Pat. No. 4,235,957, and so on.
However, the above-described method suffers from disadvantages in that it requires an extra step and materials for accelerating the bleach of dyes, specifically the superposition of an activator sheet, the heating thereof and so on. Further, the color images obtained are gradually reduced and bleached by the liberated silver which is also present in the color image upon long-range storage.
A still another method in which color images are formed by taking advantage of leuco dyes is described in, e.g., U.S. Pat. Nos. 3,985,565 and 4,022,617. However, this method has the disadvantage in that it is difficult to stably incorporate the leuco dyes in photographic materials and, consequently, the photographic material is gradually stained upon storage.
In addition, all of the above-described methods suffer from the disadvantages in that they usually require a comparatively long time for the completion of development, and the images obtained have high fog density and low image density.
For the purpose of making improvements in the above methods, methods in which silver halide is utilized and mobile dyes formed with an imagewise distribution are transferred into a dye-fixing layer are disclosed in European Patent Applications (OPI) 76,492 and 79,056 and Japanese Patent Applications (OPI) 149046/83 and 149047/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). These methods involve the transfer of imagewise formed mobile dyes into a dye-fixing layer under high temperature. However, under such a condition the development of silver halide also proceeds simultaneously. Therefore, the optimum condition allows for a narrow range of choice, and the heating under a strengthened condition causes considerable increase in fog density, generation of turbidity in dye images, and so on.